DynamicsShowcase

Everything UIKit Dynamics can do, in one app. UIKit has shipped with a 2D physics engine since iOS 7 — UIDynamicAnimator — and almost nobody uses it. This project puts every behavior it has on screen: gravity, collisions, snaps, attachments, pushes, all ten force fields, and a few things the API doesn't offer out of the box (a rope that goes slack, a hit-stop, orbits on real inverse-square gravity).

Every item on every screen is a plain UIView. No SpriteKit, no game engine, no dependencies, no assets — everything is drawn in code.

It is meant to be read as much as run: each screen's view controller opens with a comment on the part of the API it demonstrates.

Running

Open DynamicsShowcase.xcodeproj in Xcode → Run (⌘R). iOS 15+.

To jump straight to a screen, set the AUTO_OPEN_DEMO environment variable to 0…7 in the scheme — the app opens that demo on launch.

Screens

🍎 Gravity · 🧲 Snap · 🏗️ Old but gold · 🎱 Push Impulses · 🌀 Force Fields · 🪐 Solar System · ⚖️ Body Properties · 🎉 Paywall Drop

Gravity

UIGravityBehavior (vector steered by finger, magnitude slider down to a zero-g freeze), UICollisionBehavior + translatesReferenceBoundsIntoBoundary, UICollisionBehaviorDelegate (flashes + haptics), elliptical collision bounds (collisionBoundsType = .ellipse).

Code: Demos/Gravity/

Snap

UISnapBehavior with adjustable damping.

Code: Demos/Snap/

Old but gold

UIAttachmentBehavior and a custom UIDynamicBehavior. A dense "UIKit" ball hangs on a real rope — RopeBehavior: slack it does nothing, stretched it is a damped spring; drawn as a sagging Verlet chain — and smashes a wall of bricks reading "UIKit is dead", "Use SwiftUI", "Legacy"… Drag it on a springy attachment (pull past the rope's reach and it slings back) or flick to throw. A hard hit shatters a brick into snapshot shards, sends out a shockwave, shakes the screen and freezes the scene for a beat (hit-stop). A cleared wall drops a payoff line in on a UISnapBehavior and is rebuilt with a fresh draw of words.

Code: Demos/WreckingBall/ — the wrecking ball

Push Impulses

UIPushBehavior — .instantaneous (billiards-style slingshot: force = pull distance, spin via setTargetOffsetFromCenter) and .continuous with a rotating vector; a white cue ball, six pockets that pot the balls, a cleared table respawns the rack; the "Real UI" mode swaps the rack for a live settings screen (labels, cards, a working UISwitch, buttons): every cell hangs on a spring (UIAttachmentBehavior with frequency/damping), weighs in proportion to its area, squashes on contact and shatters into snapshot shards under a hard hit.

Code: Demos/Push/

Force Fields

UIFieldBehavior — all 10 types: radial, spring, vortex, noise, turbulence, velocity, linear, drag, electric, magnetic (charge via UIDynamicItemBehavior.charge), UIRegion.

Code: Demos/Fields/

Solar System

radialGravityField(falloff: 2) — a true inverse-square gravity well; planets on calibrated circular orbits (linearVelocity(for:), updateItem(usingCurrentState:)).

Code: Demos/SolarSystem/

Body Properties

UIDynamicItemBehavior — elasticity, density, resistance compared side by side, a floor boundary via addBoundary(withIdentifier:from:to:).

Code: Demos/Properties/

Paywall Drop

any UIView is a dynamic item: a realistic paywall falls off the screen under gravity on "Continue" (addLinearVelocity scatter), a UISnapBehavior greeting slowed by resistance, CAEmitterLayer confetti.

Code: Demos/Paywall/

Architecture

Lightweight MVVM, adapted to the nature of UIKit Dynamics. The physics engine animates views directly, so the view controllers own the animator and the behaviors — hiding that behind bindings would only obscure the API this project exists to demonstrate. Everything else is pulled out of the controllers:

• View models (one per screen) hold the scene configuration (tuning constants, texts) and all the pure math — layout geometry, impulse conversion, target points. They never touch views, behaviors or the animator, so they are trivially unit-testable. The clearest example is SolarSystemDemoViewModel, which turns the field's calibrated strength into circular-orbit velocities.
• Views (BallView, BoxView, DemoCardCell, …) only draw themselves.
• View controllers wire gestures to view-model math and feed the results into UIKit Dynamics behaviors. Each controller's doc comment explains which part of the API the screen demonstrates.
• DemoCatalog is the single model of the demo list; MenuViewModel serves it to the menu.

DynamicsShowcase/
├── App/                    AppDelegate, SceneDelegate
├── Core/                   Theme, Haptics, item views, shared geometry helpers,
│                           DemoViewController (base class: gradient background,
│                           animator, reset button, collision response)
├── Menu/                   DemoCatalog (model), MenuViewModel, MenuViewController, DemoCardCell
└── Demos/
    ├── Gravity/            GravityDemoViewModel + ViewController
    ├── Snap/               SnapDemoViewModel + ViewController
    ├── WreckingBall/       WreckingBallDemoViewModel (wall geometry, tuning) + ViewController, RopeBehavior, VerletRope
    ├── Push/               PushDemoViewModel + ViewController
    ├── Fields/             FieldCatalog (FieldKind + FieldFactory), FieldsDemoViewModel + ViewController
    ├── SolarSystem/        SolarSystemDemoViewModel (orbital math) + ViewController
    ├── Properties/         PropertiesDemoViewModel + ViewController
    └── Paywall/            PaywallDemoViewModel + ViewController

License

MIT — see LICENSE.